2378

OPTICS LETTERS / Vol. 36, No. 12 / June 15, 2011

Observation of stimulated Brillouinscattering in polymer
optical fiber with pump–probe technique
Yosuke Mizuno,1,* Masato Kishi,2 Kazuo Hotate,2 Takaaki Ishigure,3 and Kentaro Nakamura1
1Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,Midori-ku, Yokohama 226-8503, Japan2
Department of Electrical Engineering and Information Systems, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan3
Department of Applied Physics and Physico-Informatics, Keio University, 3-14-1, Hiyoshi,Kohoku-ku, Yokohama 223-8522, Japan*Corresponding author: ymizuno@sonic.pi.titech.ac.jp
Received April 20, 2011; accepted May 17, 2011;
posted May 18, 2011 (Doc. ID 146210); published June 15, 2011
Stimulated Brillouin scattering (SBS) in a perfluorinated graded-index polymer optical fiber (POF) with 120 μm
core diameter was experimentally observed for the first time, to the best of our knowledge, at 1:55 μm wavelengthwith the pump–probe technique. Compared to spontaneous Brillouin scattering previously reported, the Brillouingain spectrum (BGS) was detected with an extremely high signal-to-noise ratio, even with a short POF (1 m) andscrambled polarization state. We also investigated the BGS dependences on probe power and temperature, whichindicate that SBS in a POF measured with this technique can be utilized to develop high-accuracy temperaturesensing systems. © 2011 Optical Society of America
OCIS codes: 130.5460, 290.5830.

Brillouin scattering in silica-based optical fibers has been
extensively studied [1,2] and applied to a large numberof useful devices and systems, such as lasers [2,3], optical
comb generators [3], microwave signal processors [4],slow light generators [5], phase conjugators [6], andstrain/temperature sensors [7–11]. To improve the performance of these systems, Brillouin scattering in various
specialty fibers, including tellurite glass fibers [12,13],chalcogenide fibers [14,15], bismuth-oxide fibers [16,17],and photonic crystal fibers [18,19] has been investigatedso far. Recently, we have characterized the spontaneous
Brillouin scattering (SpBS) at 1:55 μm in polymer opticalfibers (POFs) [20], which offer extremely high flexibility,
easy and low-cost connection, and high safety comparedto other standard glass fibers [21]. The estimatedBrillouin gain coefficient of 3:09 × 10−11 m=W was comparable to that of a silica single-mode fiber (SMF), indicating that Brillouin scattering in POFs can be applied
to a variety of practical devices and systems in the same
way as Brillouin scattering in silica fibers. We have also
investigated the dependences of the Brillouin frequency
shift (BFS) on strain and temperature in a POF at 1:55 μmand found that SpBS in POFs can be potentially used
to develop strain-insensitive high-accuracy temperature
sensors [22].
The Brillouin scattering in POFs observed in the previous experimental setup [20,22] was not stimulated but
spontaneous because the Brillouin threshold of POFs
was estimated to be as high as 24 W due to their multimode nature [20]. Consequently, the power of the reflected Stokes light was so low that the following four
problems arose: (1) a POF longer than several meters
was needed, (2) the polarization state had to be optimized, (3) averaging of the spectral data had to be performed several tens of times, and (4) the signal-to-noise(S/N) ratio of the Brillouin gain spectrum (BGS) was very
0146-9592/11/122378-03$15.00/0

low, even when (1), (2), and (3) were cleared. In order toimplement practical Brillouin sensors and other systems
using POFs, these problems need to be resolved.
In this Letter, we report on the observation of stimulated Brillouin scattering (SBS) in a POF with 120 μm
core diameter at 1:55 μm wavelength with the pump–probe technique. The BGS is detected with an extremely
high S/N ratio, even with a 1 m POF, scrambled polarization state, and no averaging. We also investigate theBGS dependences on probe power and temperature andconfirm that SBS in a POF measured with this technique
can be utilized to develop high-accuracy temperature
sensors.
When pump light is injected into one end of a fiber
under test (FUT), weak reflected light, called Stokes
light, is generated due to SpBS with the center frequency
downshifted by the BFS of several gigahertz [1,2]. If thepower of the pump light is higher than the Brillouinthreshold, the Stokes light caused by SpBS acts as a seedof stimulated scattering, and there occurs a transition
from SpBS to SBS. As a result, the power of the Stokes
light is drastically enhanced [2]. On the other hand, whenprobe light at the same frequency as the Stokes light isalso injected into the other end of the FUT, SBS is induced even when the power of the pump light is much
lower than the Brillouin threshold because the probe
light itself acts as a seed of stimulated scattering [23].
This “pump–probe” technique has been used to develop
Brillouin systems with a high S/N ratio [9].
The FUT used in the experiment was a 1 m CYTOPbased perfluorinated graded-index POF [24] with an
NA of 0.185, a core diameter of 120 μm, a core refractive
index of ∼1:35, and a propagation loss of ∼150 dB=km at
1:55 μm. The experimental setup shown schematicallyin Fig. 1 is similar to that of Brillouin optical correlationdomain analysis [8,9]. The light beam from a 1550 nm© 2011 Optical Society of America

June 15, 2011 / Vol. 36, No. 12 / OPTICS LETTERS

Fig. 1. (Color online) Experimental setup for observing SBS
in POF with pump–probe technique. DAQ, data acquisition;
DC, direct current; LD, laser diode; EDFA, erbium-doped fiber
amplifier; FG, function generator; FUT, fiber under test; IM,intensity modulator; LI-A, lock-in amplifier; MG, microwave
generator; OSC, oscilloscope; PC, personal computer; PD,photodetector; POF, polymer optical fiber; PSCR, polarization
scrambler; SSBM, single-sideband modulator; VOA, variable
optical attenuator.

three-electrode laser diode was divided into two. Onewas used as the pump light, after being chopped withan intensity modulator for lock-in detection and being
amplified with an erbium-doped fiber amplifier (EDFA).
The other was used as the probe light, after passing
two EDFAs, a single-sideband modulator (SSBM), and apolarization scrambler (PSCR). The SSBM was employedwith a microwave generator (MG) and a proper DC bias
control to suppress the carrier (pump) and the antiStokes component of the two first-order sidebands andto maintain a stable frequency downshift from the pumplight [25]. This frequency downshift was swept from 2.5
to 3:5 GHz with a period of 300 ms to obtain the BGS ofthe POF, which is observed approximately at 2:8 GHz.
The suppression ratio of the other frequency components
was kept higher than 25 dB, as shown in Fig. 2. The PSCR,
which can modulate the polarization state at 1 MHz, wasinserted to suppress the polarization-dependent fluctuations of the signal [26]. The POF and the silica SMFs werebutt-coupled with the gaps filled with index-matching oil

Fig. 2. (Color online) Measured optical spectrum of the SSBM
output when the frequency of the MG was set to 2:83 GHz.

Fig. 3.

2379(Color online) BGS in POF observed without averaging.

(n ¼ 1:46) to minimize the Fresnel reflection [20]. The
Stokes light was adjusted in power with a variable opticalattenuator and converted to an electrical signal with a
125 MHz photodetector. After passing a lock-in amplifier(LI-A) with a chopping frequency of 5:018 MHz and a time
constant of 10 ms, the electrical signal was observed asa BGS with an oscilloscope synchronized with the frequency sweep of the SSBM.
Figure 3 shows the measured BGS with no averaging
when the pump power and the probe power were 23 and22 dBm, respectively. The power was normalized so thatthe peak power was 1.0. Although the POF length wasonly 1 m and the polarization state was scrambled, theBGS was observed with a much higher S/N ratio than that
previously reported [22]. The BFS was 2:86 GHz, whichis slightly higher than the previously reported value of2:83 GHz [20]. This discrepancy seems to originate fromthe difference in temperature and the time constant ofthe LI-A, which is not short enough. The 3 dB bandwidthof the BGS measured in this experiment was about
160 MHz, but further research is needed on the bandwidth because it is also dependent on the time constantof the LI-A (when the time constant was shorter than
10 ms, the BGS was distorted).
Figure 4 shows the dependence of the BGS on probe
power when the pump power was fixed at 23 dBm. TheFig. 4. (Color online) Dependence of BGS on probe powerin POF.

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OPTICS LETTERS / Vol. 36, No. 12 / June 15, 2011
References

Fig. 5. (Color online) Dependence of (a) BGS and (b) BFS on
temperature in POF.

probe power was reduced from 22 to 6 dBm, and averaging was performed 30 times. As the probe power
decreased, the Stokes power also decreased, which
proves that this BGS is caused by the interaction between
the pump light and the probe light, i.e., SBS.
We also measured the dependence of the BGS on
temperature as shown in Fig. 5(a). The pump power andthe probe power were 23 and 22 dBm, respectively, andaveraging was performed 30 times. The temperature
was set to 20 °C, 40 °C, and 60 °C. With the increasing
temperature, the BGS shifted toward lower frequency.
Figure 5(b) shows the temperature dependence of theBFS. The slope of −4:05 MHz=K is in good agreementwith the previous report [22], which confirms that theBGS in a POF observed with the pump–probe technique
can be applied to high-accuracy temperature sensing.
In conclusion, we observed the SBS in a POF at
1:55 μm with the pump–probe technique. We successfully
detected the BGS with an extremely high S/N ratio, evenwith a 1 m POF, scrambled polarization state, and noaveraging. We also investigated the BGS dependence
on probe power, which proved that the measured BGS
was caused not by SpBS but by SBS. Besides, we measured the BGS dependence on temperature, showing thatthe BGS in a POF observed with this technique can beapplied to temperature sensing with high accuracy. We
believe that this work is a significant step toward the development of Brillouin-based distributed temperature/strain sensing systems using POFs.
The work of Y. Mizuno was supported by the Research
Fellowships for Young Scientists from the Japan Society
for the Promotion of Science (JSPS).

1. E. P. Ippen and R. H. Stolen, Appl. Phys. Lett. 21, 539(1972).
2. G. P. Agrawal, Nonlinear Fiber Optics (Academic, 1995).
3. G. J. Cowle, D. Yu, and Y. T. Chieng, J. Lightwave Technol.
15, 1198 (1997).
4. S. Norcia, S. Tonda-Goldstein, D. Dolfi, and J.-P. Huignard,Opt. Lett. 28, 1888 (2003).
5. K. Y. Song, M. G. Herraez, and L. Thevenaz, Opt. Express
13, 82 (2005).
6. E. A. Kuzin, M. P. Petrov, and B. E. Davydenko, Opt. Quantum Electron. 17, 393 (1985).
7. T. Horiguchi and M. Tateda, J. Lightwave Technol. 7, 1170
(1989).
8. K. Hotate and T. Hasegawa, IEICE Trans. Electron. E83-C,405 (2000).
9. K. Y. Song and K. Hotate, IEEE Photon. Technol. Lett. 18,499 (2006).
10. D. Garus, K. Krebber, F. Schliep, and T. Gogolla, Opt. Lett.
21, 1402 (1996).
11. Y. Mizuno, W. Zou, Z. He, and K. Hotate, Opt. Express 16,12148 (2008).
12. K. S. Abedin, Opt. Express 14, 11766 (2006).
13. Y. Mizuno, Z. He, and K. Hotate, Opt. Commun. 283, 2438
(2010).
14. K. S. Abedin, Opt. Express 13, 10266 (2005).
15. K. Y. Song, K. S. Abedin, K. Hotate, M. G. Herraez, andL. Thevenaz, Opt. Express 14, 5860 (2006).
16. J. H. Lee, T. Tanemura, K. Kikuchi, T. Nagashima, T.
Hasegawa, S. Ohara, and N. Sugimoto, Opt. Lett. 30, 1698
(2005).
17. Y. Mizuno, Z. He, and K. Hotate, Appl. Phys. Express 2,112402 (2009).
18. J. H. Lee, Z. Yusoff, W. Belardi, M. Ibsen, T. M. Monro, and
D. J. Richardson, Opt. Lett. 27, 927 (2002).
19. J. C. Beugnot, T. Sylvestre, D. Alasia, H. Maillotte, V. Laude,A. Monteville, L. Provino, N. Traynor, S. F. Mafang, and
L. Thevenaz, Opt. Express 15, 15517 (2007).
20. Y. Mizuno and K. Nakamura, Appl. Phys. Lett. 97, 021103(2010).
21. M. G. Kuzyk, Polymer Fiber Optics: Materials, Physics,and Applications (CRC Press, 2006).
22. Y. Mizuno and K. Nakamura, Opt. Lett. 35, 3985 (2010).
23. N. Shibata, Y. Azuma, T. Horiguchi, and M. Tateda, Opt.
Lett. 13, 595 (1988).
24. T. Ishigure, Y. Koike, and J. W. Fleming, J. Lightwave
Technol. 18, 178 (2000).
25. S. Shimotsu, S. Oikawa, T. Saitou, N. Mitsugi, K. Kubodera,
T. Kawanishi, and M. Izutsu, IEEE Photon. Technol. Lett.
13, 364 (2001).
26. Y. Mizuno, Z. He, and K. Hotate, Appl. Phys. Express 2,062403 (2009).